EVIDENCE FOR EARLY PLEISTOCENE GLACIATION SOUTH OF THE MISSOURI RIVER IN ST. LOUIS COUNTY, MISSOURI: IMPLICATIONS FOR TILL STRATIGRAPHY EAST OF THE MISSISSIPPI RIVER

Many unpublished or otherwise obscure reports mention scattered remnants of glacial till in St. Louis County, Missouri, even though this is south of the traditional placement of the pre-Illinoian boundary. Following prior usage we refer to this deposit informally as the “Mill Creek till“. The Mill Creek has not been dated directly, but it is very similar to the ~2.4 Ma Atlanta Fm. (the oldest dated till in Missouri), which is present just a few km to the northwest. Highly weathered material from the Mill Creek is incorporated into overlying deposits dated at earliest Middle Pleistocene; thus, the Mill Creek till probably is Early Pleistocene in age.

Both the Atlanta and Mill Creek tills have much higher concentrations of chert clasts than younger tills in Missouri, which date from ~1.3 - ≤ 0.4 Ma, and these two have nearly identical matrix texture and sand lithology, which are distinct from all other local tills. Moreover, both the Atlanta and Mill Creek have high ratios of illite:expandable clay and garnet:epidote grains, implying ice flow from a far easterly (Labradoran) location. Nevertheless, the Mill Creek has a higher content of quartzite clasts, along with a lower percentage (and different types) of igneous. The Atlanta till nearly lacks oolitic chert, but this is common within the Mill Creek, as are well-rounded clasts with patches of relict silica cement encasing smaller detrital grains. Thus, the Mill Creek till was partly derived from conglomeritic sandstones. The Mill Creek also has higher percentages of unstable (mafic) minerals within the sand fraction, indicating that the ice traversed either a different or less-weathered area of the Canadian Shield.

Based on the inferred provenance and flow direction, both of these tills should be present east of the Mississippi River, although they may be difficult to recognize as tills due to their low content of igneous erratics. Finally, many of the chert and quartzite clasts within the Mill Creek seem identical to those within conglomerates of purported Cretaceous age that are preserved locally across the Midwest. The concentration of these clasts within the Mill Creek implies a widespread presence of such conglomerates up to at least the Early Pleistocene.